Mitochondrial dysfunction is the major mechanism precipitating I/R injury which commonly occurs during liver surgery, trauma, hemorrhagic shock and liver transplantation. Sirtuin 1 (SIRT1) is an NAD+-dependent deacetylase that induces longevity, stress resistance and tumor suppression. The role of SIRT1 in ischemia/reperfusion-mediated liver injury is unknown. The goal of this study is to investigate the role of SIRT1 in I/R injury to liver and to develop therapeutic strategies to improve liver function after I/R. Our principal hypothesis is that calpain-dependent SIRT1 loss causes a sequential chain of defective mitophagy, mitochondrial permeability transition (MPT) onset and hepatocyte death after I/R. Accordingly, we propose that restoration or enhancement of hepatic SIRT1 will promote mitophagy and consequently ameliorate mitochondrial failure and liver dysfunction after reperfusion. To test our hypothesis, we will use hepatocytes isolated from SIRT1 wild type (WT) and knockout (KO) mice for characterization of cellular mechanisms causing SIRT1 depletion, defective mitophagy, and onset of the MPT and cell death after I/R. In addition, we will use anesthetized WT and KO SIRT1 mice to confirm and extend our in vitro findings to an in vivo model of hepatic I/R. Finally, we will extend and translate our findings from mice into human liver biopsies. These studies provide critical mechanistic insights into lethal I/R injury to the lver, and will establish novel therapeutic approaches for improving I/R-mediated liver failure.